1. Field of the Invention
The present invention is in the field of manufacture of layers of silicides of high melting point metals onto substrates of silicon and/or silicon dioxide particularly employed in semiconductor technology for VLSI (very large scale integration)-circuits. The method involves thermal decomposition of silicon containing hydrogen compounds, or halogenated silanes, and metal halides and deposition of the resulting metal silicides from the gas phase at reduced pressure onto the substrate.
2. Description of the Prior Art
The deposition of tantalum silicide from a gaseous mixture at low pressures has been described in an article by Lehrer in the Proceedings of the 1st International Symposium on VLSI Science and Technology (1982), pages 258 to 264, as well as from U.S. Pat. No. 4,359,490.
The VLSI-Technology requires, in the manufacture of interconnect metallization planes, very small (one square micron or so) and deep (one micron) contact holes. With previously employed physical coating methods, such as evaporation or sputtering, the patterned substrate wafer can only be insufficiently coated at steps with the deposited silicides such as the disilicides of the metals titanium, tantalum, molybdenum, tungsten or cobalt. A good step coverage can only be obtained if the manufacture of the layer is accomplished by the proven method of chemical vapor deposition (CVD). To obtain a satisfactory step coverage in evaporation or sputtering equipments, substrate holders must be employed whereby the substrates in addition to being given a planetary motion, must also be given a tumbling motion in order that the steam jet will strike the substrates at angles of incidence which are as large as possible and which are different. These mechanical apparatus produce very small particles which lead to defects in the microstructured components. For this reason, a coating method having good step coverage is of great advantage. The CVD method is such a method since the material is deposited from the gas phase and a patterned substrate is uniformly coated.
In the known process of deposition of polycrystalline silicon according to the pyrolytic decomposition of silane: EQU SiH.sub.4 .fwdarw.Si+2H.sub.2 ( 1)
A reduction of the reaction pressure (starting with normal pressure) brings about a significant improvement regarding homogeneity in the layer thickness over the substrate surface even if the wafers are tightly packed (see K. F. Jensen, D. B. Graves, J. Electrochem. Soc., Vol. 130, No. 9 (1983), pages 1950 to 1957).
From the initially cited article of Lehrer, the following process parameters are known for the deposition of polycrystalline silicon:
A temperature of 615.degree. to 635.degree. C., a pressure of 0.3 Torr (approximately 0.39 mbar), and a silane quantity of 30 standard cc/min.
If it is desired to manufacture a low resistance metal silicide (for example, tantalum silicide) according to the process parameters used in the case of the deposition of polycrystalline silicon, through admixture, for example, of a metal halide such as tantalum chloride, difficulties arise regarding homogeneity in the layer thickness and material composition. The cause for this is a separation of the reaction gases (silane and tantalum chloride) due to the greatly different molecular weights, as well as the onset of secondary reactions such as, for example: EQU SiH.sub.4 +TaCl.sub.5 .fwdarw.Ta+SiCl.sub.4 +3/2 H.sub.2 +HCl (2)
From the article by Lehrer, it is further known that the homogeneity in the layer thickness distribution and/or material composition of the tantalum silicon layers are very strongly dependent upon the reactor pressure and that the uniform coating of several silicon substrates with, for example, tantalum disilicide, apparently presents difficulties. If, simultaneously, for example, 25 pieces of 3-inch silicon wafers are to be coated, which substrates are covered with a native oxide layer of approximately 3 nm, the process according to Lehrer must be varied such that initially a silicon layer is deposited and, thereupon, a tantalum-rich silicide layer is produced according to the following gross reaction: EQU 3 SiH.sub.4 +5 TaCl.sub.5 +13/2 H.sub.2 .fwdarw.Ta.sub.5 Si.sub.3 +25 HCl (3)
In order to obtain TaSi.sub.2, this Si/Ta.sub.5 Si.sub.3 double layer must be briefly annealed, for example, at 800.degree. C. For the deposition of tantalum silicon, the following parameters have been expressed in the article of Lehrer:
A temperature of 615.degree. to 635.degree. C., a pressure of 0.28 Torr (approximately 0.37 mbar) a silane quantity of 24 sccm, a tantalum chloride temperature of 125.degree. C., corresponding to a partial pressure of approximately 3 Torr (approximately 4 mbar) and a hydrogen rate of 5 sccm. The process proposed by Lehrer (hot wall reactor, pressure range: 0.2 to 0.3 Torr) implies a preheating of the reaction gases thus favoring the homogeneous nucleation of Ta in the gas phase according to equation 2. Further on hot parts of the quartz tube (about 620.degree. C.) were coated with Ta.sub.5 Si.sub.3 according to equation 3. A reason for the variation of the sheet resistance (about .+-.10%) and its strong dependence from the reaction pressure may be that the decomposition products produced by the above-mentioned reactions may not be removed quickly enough thus influencing the deposition at the substrate position.
In order to reduce the relative amount of decomposition products, taking into account lesser quantities per charge, it is possible as disclosed in an article by D. L. Brors et al in Solid State Technology (April 1983), pages 183 to 186, for the deposition of tungsten silicide, to introduce a cold wall reactor and additionally dilute the reaction mixture with an inert gas. This proceeds according to the gross reaction equation: ##STR1##
This reaction is carried out at a temperature of 350.degree. to 400.degree. C. and at a pressure of 0.050 to 0.300 Torr (approximately 0.066 to 0.395 mbar).
Finally, it is known from an article by K. Akitmoto and K. Watanabe appearing in the Appl. Phys. Letters 39 (September 1981), pages 445 to 447, that the reaction of silane and tungsten fluoride can produce W.sub.x Si.sub.1-x by chemical vapor deposition when enhanced with plasma (PECVD), the temperature amounting to 230.degree. C., and the pressure being from 0.5 to 0.7 Torr (0.66 to 0.92 mbar).
The drawback of all the CVD-methods is that the homogeneity in relation to the layer thickness and material composition as well as the free selectability of layer compositions is not optimum in the case of manufacture of metal-silicon-alloy layers with a high throughput because either decomposition products increase in amount or the reactants become separated.